Demand for efficient and reliable electrical power is escalating, outpacing improvements in conventional power sources. In addition, concerns about global climate change are increasing with rising the level of CO2 in our atmosphere, caused by the use of combustion-based methods to generate power from fossil fuels. Fuel cells offer a viable approach to increasing efficiency of power generation from fossil fuels, while greatly reducing emissions of greenhouse gases and other pollutants.
Of the various types of fuel cells, the proton exchange membrane (PEM) fuel cell, which operates with hydrogen as a fuel, is receiving considerable attention due to its low weight, low-temperature operation, and ease of manufacture. However, operation of PEM fuel cells with fossil-based hydrocarbon fuels requires extensive pre-processing (reforming) to convert the hydrocarbons into a hydrogen rich gas, and subsequent gas purification steps to reduce carbon monoxide and sulfur to very low levels (CO<10 ppm and H2S<10 ppb).
Solid oxide fuel cells (SOFCs) operate at high temperature (typically, 600 to 1000° C.) and are much less sensitive to impurities in the hydrocarbon fuels, which minimizes the amount of gas purification steps required. This greatly increases power generation efficiency and reduces system complexity. It also is possible to operate SOFCs directly on certain hydrocarbon fuels (e.g., methane, methanol and ethanol) via internal reforming, i.e., without an initial reforming step.
A simplified schematic of a repeat unit of a planar SOFC stack is shown in FIG. 1. As seen in FIG. 1, a solid oxide fuel cell typically comprises an oxygen ion conducting ceramic electrolyte membrane that is sandwiched between a fuel electrode (anode) and an air electrode (cathode). Power is generated by passing air (or oxygen) over the cathode and fuel (e.g., hydrogen plus carbon monoxide) over the anode, with the fuel cell maintained at an elevated temperature (e.g., 600 to 1000° C.). Oxygen reduction occurs at the cathode, and the oxygen ions are conducted through the electrolyte to the anode. At the anode, the oxygen ions oxidize the hydrogen in the fuel, which generates electrical current, along with steam and carbon dioxide. Ceramic electrolyte materials used in SOFCs can include, for example, yttrium-stabilized zirconia (YSZ), scandia-stabilized zirconia (ScSZ), zirconium oxide doped with any combination of rare earth and/or alkaline earth elements, samarium-doped ceria (SDC), gadolinium-doped ceria (GDC), cerium oxide doped with any combination of rare earth and/or alkaline earth elements, lanthanum strontium magnesium gallium oxide (LSGM), and other oxygen-ion-conducting ceramic electrolyte materials known to those skilled in the art. It should be pointed out that the term “air,” as used herein, and unless otherwise indicated, is intended to encompass any oxygen-containing gas stream suitable for use with SOFCs, including pure oxygen.
Two of the keys to successful commercial development of SOFC systems are the electrochemical cell design and the “stacking” configuration (i.e., the manner in which the individual fuel cells and related components are configured within a power producing SOFC module or stack of cells). For example, it is often important to pack as large amount of active area for electrochemical reactions as possible within the smallest volume possible. A typical solid oxide fuel cell will generate about 20 to 40 watts of power for every 100 cm2 of active cell area—this translates to more than about 3000 cm2 of active area for a kilowatt of power.
Planar SOFCs have been demonstrated extensively. Typically, the fuel cell (anode/electrolyte/cathode layers) is mechanically supported by the anode, requiring that the anode layer be the thickest of the three. In other designs, the fuel cell is supported by the electrolyte layer, which typically necessitates a thick electrolyte layer which can impede performance.
Recently, novel fuel cells incorporating self-supporting electrolyte membranes have been developed by NexTech Materials, Ltd. (Lewis Center, Ohio) which overcome some of the technical barriers associated with building SOFC stacks with conventional planar cells. Some of these electrolyte membrane and fuel cell designs are described, for example, in published patent application US 2006/0234100 A1, published on Oct. 19, 2006, titled “Self-Supporting Ceramic Membranes and Electrochemical Cells and Cell Stacks Including the Same,” and in U.S. Pat. No. 7,736,787 B2, issued Jun. 15, 2010, titled “Ceramic Membranes With Integral Seals and Support, and Electrochemical Cells and Electrochemical Cell Stacks Including the Same.” The aforementioned published patent application and issued patent are each incorporated by reference herein.
In one such design marketed by NexTech Materials as the FlexCell™ fuel cell, the electrolyte membrane includes a thin electrolyte layer that is mechanically supported by a “honeycomb” mesh layer of electrolyte material (as further described in U.S. Pub. No. 2006/0234100). In the FlexCell™ fuel cell design, more than 75 percent of the electrolyte membrane within the active area may be thin (20 to 40 microns), and the periphery of the cell is dense. Electrode (anode and cathode) layers are subsequently deposited onto the major faces within the active cell regions to complete the fabrication of the SOFC. As used herein, the term “dense” means that there is substantially no interconnected porosity and substantially no gas permeability.
An alternative design marketed by NexTech Materials is the HybridCell™ fuel cell. While the self-supporting electrolyte membrane includes a thin electrolyte layer that is mechanically supported by a “honeycomb” mesh layer of electrolyte material (as further described in U.S. Pat. No. 7,736,787), the anode layer (e.g., 30 to 40 microns thick) is co-sintered between the mesh support layer and the thin electrolyte membrane layer. The cathode layer is deposited on an outer surface of the electrolyte membrane within the active cell region, such as on the outer surface of the thin electrolyte layer. In the HybridCell™ fuel cell, the entire active cell area has a thin electrolyte membrane layer (e.g., 10 to 20 microns thick), and the periphery of the cell is dense.
Further details regarding the FlexCell™ and HybridCell™ fuel cell designs, as well as other forms of electrolyte-supported fuel cells and various ways of providing anode and cathode layers on such fuel cells, are described in U.S. published patent applications 2009/0148742 A1 and 2009/0148743 A1, both of which were published on Jun. 11, 2009, and are titled “High Performance Multilayer Electrodes for Use in Reducing Atmospheres.” These two published applications are also incorporated by reference herein.
In order to generate useful amounts of electrical power, planar SOFCs are usually configured in a “stack”, with multiple planar fuel cells separated by planar electrical interconnect components (also referred to as “interconnects”) that conduct electricity between the cells. Typically, and as shown in FIG. 1, the interconnects also define the flow paths for oxidant (air or oxygen) through the cathode channels and fuel (H2, CO, CH4, etc.) through the anode channels.
Many planar stack designs also include conductive foams or meshes within the stack to facilitate current collection. For example, a cathode current collector is sometimes positioned between, and in electrical contact with, the interconnect and the cathode face of the planar SOFC cell, and an anode current collector is positioned between, and in electrical contact with, the interconnect and the anode face of the planar SOFC cell. It is also important to provide gas-tight seals between the anode and cathode chambers of the stack for efficient fuel cell operation.
While a variety of SOFC stack designs and components may exist, it is believed that no one prior to the inventors has made or used an invention as described herein.
The drawings are not intended to be limiting in any way, and it is contemplated that various embodiments of the invention may be carried out in a variety of other ways, including those not necessarily depicted in the drawings. The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention; it being understood, however, that this invention is not limited to the precise arrangements shown.